The recent letter from Iranian President Mahmoud Ahmadinejad to President Bush needs to be considered on several levels. It can be treated as a ploy to obstruct U.N. Security Council deliberations on Iran's disregard of its obligations under the Non-Proliferation Treaty. This consideration, and the demagogic tone of the letter, merited its rejection by Secretary of State Condoleezza Rice. But the first direct approach by an Iranian leader to a U.S. president in more than 25 years may also have intentions beyond the tactical and propagandistic, and its demagoguery may be a way to get the radical part of the Iranian public used to dialogue with the United States. America's challenge is to define its own strategy and purposes regarding the most fateful issue confronting us today.

The world is faced with the nightmarish prospect that nuclear weapons will become a standard part of national armament and wind up in terrorist hands. The negotiations on Korean and Iranian nuclear proliferation mark a watershed. A failed diplomacy would leave us with a choice between the use of force or a world where restraint has been eroded by the inability or unwillingness of countries that have the most to lose to restrain defiant fanatics. One need only imagine what would have happened had any of the terrorist attacks on New York, Washington, London, Madrid, Istanbul or Bali involved even the crudest nuclear weapon.

Of the two negotiations, the one on Korea -- a six-party forum of Japan, South Korea, China, the United States, Russia and North Korea -- seems more advanced than the four-party talk on Iran (among France, Germany, Britain and Iran). Last September an apparent agreement in principle was reached in Beijing that North Korea will give up its nuclear program if the other parties provide adequate assurances of security, economic help in the post-nuclear period and a substitute for the power generation allegedly lost by abandoning the nuclear program. But each side has demanded that the other fulfill all its obligations before it undertakes its own; a serious effort to discuss a concurrent schedule has been prevented by North Korea's tactic of stringing out the period between each session, perhaps to gain time for strengthening its nuclear arsenal.

With respect to Iran, there isn't even a formal agreement on what the objective is. Iran has refused to agree to international control over its uranium enrichment program, in the absence of which no control over a weapons program is meaningful.

The public debate often focuses on whether the United States is prepared to engage in bilateral discussions with North Korea or Iran. With respect to Korea, that is a subsidiary issue. The six-power talks provide adequate opportunity for a bilateral exchange of views. What Pyongyang is attempting to achieve -- and what the Bush administration has rightly resisted -- is a separate negotiation with Washington outside the six-party framework, which would prevent other parties in the Beijing process from undertaking joint responsibilities. If bilateral talks replaced the six-party forum, some of America's present partners might choose to place the onus for breaking every deadlock on Washington, in effect isolating the United States.

The same considerations apply even more strongly to bilateral negotiations with Iran at this stage. Until now formal negotiations have been prevented by the memory of the hostage crisis, Iranian support of terrorist groups and the aggressive rhetoric of the Iranian president. Nor does the Iranian president's letter remove these inhibitions. Nevertheless, on a matter so directly involving its security, the United States should not negotiate through proxies, however closely allied. If America is prepared to negotiate with North Korea over proliferation in the six-party forum, and with Iran in Baghdad over Iraqi security, it must be possible to devise a multilateral venue for nuclear talks with Tehran that would permit the United States to participate -- especially in light of what is at stake.

An indefinite continuation of the stalemate would amount to a de facto acquiescence by the international community in letting new entrants into the nuclear club. In Asia, it would spell the near-certain addition of South Korea and Japan; in the Middle East, countries such as Turkey, Egypt and even Saudi Arabia could enter the field. In such a world, all significant industrial countries would consider nuclear weapons an indispensable status symbol. Radical elements throughout the Islamic world and elsewhere would gain strength from the successful defiance of the major nuclear powers.

The management of a nuclear-armed world would be infinitely more complex than maintaining the deterrent balance of two Cold War superpowers. The various nuclear countries would not only have to maintain deterrent balances with their own adversaries, a process that would not necessarily follow the principles and practices evolved over decades among the existing nuclear states. They would also have the ability and incentives to declare themselves as interested parties in general confrontations. Especially Iran, and eventually other countries of similar orientation, would be able to use nuclear arsenals to protect their revolutionary activities around the world.

There is an argument on behalf of acquiescing in proliferation which holds that new nuclear countries have proved responsible in the past. But this is not endorsed by experience. Pakistan proliferated its nuclear technology through the A.Q. Khan project; North Korea has been an active proliferator. In addition, the safeguarding of nuclear material on the territories of emerging nuclear countries is bound to be more porous and less sophisticated.

Diplomacy needs a new impetus. As a first step, the United States and its negotiating partners need to agree on how much time is available for negotiations. There seems to be general agreement that Pyongyang is producing enough plutonium for several weapons a year; there is some disagreement about progress in producing actual operational weapons in the absence of testing. Estimates on how close Tehran is to producing its first nuclear weapon range from two to 10 years. Given the risks and stakes, this gap needs to be narrowed. Any consideration of diplomatic pace must take account of the fact that in 2008 governments in both Russia and the United States will change; this will impose a hiatus on diplomacy while the governments are preoccupied with transition and, in America, restaffing the executive branch.

The next step is to recognize the difference between multiparty negotiations and a preferred strategy of regime change. There are no governments in the world whose replacement by responsible regimes would contribute more to international peace and security than those governing Pyongyang and Tehran. But none of the participants in the existing or foreseeable forums will support a policy explicitly aiming for regime change. Inevitably, a negotiation on nuclear disarmament will involve compensation in security and economic benefits in return for abandonment of nuclear weapons capabilities and is, in that sense, incompatible with regime change.

Focusing on regime change as the road to denuclearization confuses the issue. The United States should oppose nuclear weapons in North Korea and Iran regardless of the government that builds them.

The diplomacy appropriate to denuclearization is comparable to the containment policy that helped win the Cold War: no preemptive challenge to the external security of the adversary, but firm resistance to attempts to project its power abroad and reliance on domestic forces to bring about internal change. It was precisely such a nuanced policy that caused President Ronald Reagan to invite Soviet leader Leonid Brezhnev to a dialogue within weeks of labeling the Soviet Union as the evil empire.

On Korea, progress requires agreement regarding the political evolution of the Korean Peninsula and of Northeast Asia. The expectation that China is so reluctant to see nuclear weapons on the Korean Peninsula -- and therefore ultimately in Japan -- that it will sooner or later bring the needed pressure on North Korea has so far been disappointed. This is because China has not only military concerns but also strategic objectives on the Korean Peninsula. It will try to avoid an outcome in Korea that leads to the sudden collapse of an ally, producing a flood of Korean refugees into China as well as turmoil on its borders. For these reasons, a strategic dialogue with Beijing must be an important component of a negotiating strategy that also addresses Pyongyang's desire for security.

Though America is represented in the six-party forum by an exceptional diplomat in Christopher Hill, periodic engagement at a higher level is needed to give the necessary direction to his efforts. The objective should be an understanding regarding security and political evolution in Northeast Asia that requires no changes in sovereignty as part of the process of denuclearization but leaves open the prospect of Korean unification through negotiations or internal evolution.

Parallel considerations apply to the case of Iran. The current negotiating forum is highly dysfunctional. Three European countries in close coordination with the United States are acting partly as America's surrogate. China and Russia do not participate in the negotiations but are involved when their consequences go before the U.N. Security Council -- a procedure enabling Iran to play off the nuclear powers against each other.

A more coherent forum for negotiation would combine the three European nations with the United States, China and Russia as the countries most directly affected and in the best position to act jointly in the Security Council. This could be set up after the passage of the Security Council resolution now under discussion. It would permit elaboration of the one hopeful scheme that has emerged in Iranian diplomacy. Put forward by Russia, it is to move certain enrichment operations out of Iran into Russia, thereby preventing clandestine weaponization. The new, broader forum could be used to establish an international enrichment program applicable to future nuclear technologies to curb the looming specter of unchecked proliferation.

Obviously, nuclear proliferation cannot be prevented simply by multiplying negotiating forums. The experience with existing conferences demonstrates the capacity for procrastination and obfuscation. To be effective, diplomacy must involve a willingness to provide clear penalties for obstruction.

Only after we have created the requisite negotiating framework and explored all aspects of diplomacy should the issue of military measures be addressed. But neither should force be rejected in principle and for all time before we know the circumstances in which this last resort should be considered.

The issue before the nations involved is similar to what the world faced in 1938 and at the beginning of the Cold War: whether to overcome fears and hesitancy about undertaking the difficult path demanded by necessity. The failure of that test in 1938 produced a catastrophic war; the ability to master it in the immediate aftermath of World War II led to victory without war.

The debates surrounding these issues will be conducted in the waning years of an American adm1inistration. On the surface, this may seem to guarantee partisanship. But thoughtful observers in both parties will know that the consequences of the decisions before us will have to be managed in a new administration. The nuclear issue, capable of destroying mankind, may thus, one hopes, bring us together in the end.